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Journal of Arid Environments 73 (2009) 202–211

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Journal of Arid Environments

journal homepage: www.elsevier .com/locate/ jar idenv

Land use and disturbance effects on the dynamics of natural ecosystemsof the Monte Desert: Implications for their management

P.E. Villagra a,b,*, G.E. Defosse c,d, H.F. del Valle e, S. Tabeni f, M. Rostagno e, E. Cesca a, E. Abraham f

a Departamento de Dendrocronologıa e Historia Ambiental, Instituto Argentino de Nivologıa, Glaciologıa y Ciencias Ambientales (IANIGLAdCONICET), 5500 Mendoza, Argentinab Facultad de Ciencias Agrarias, Universidad Nacional de Cuyo, Argentinac Centro de Investigacion y Extension Forestal Andino Patagonico (CIEFAPdCONICET), Argentinad Universidad Nacional de la Patagonia, sede Esquel, Esquel, Argentinae Ecologıa Terrestre, Centro Nacional Patagonico (CENPATdCONICET), Puerto Madryn, Argentinaf Instituto Argentino de Investigaciones de las Zonas Aridas (IADIZAdCONICET), Mendoza, Argentina

a r t i c l e i n f o

Article history:Received 20 March 2007Received in revised form24 July 2008Accepted 1 August 2008Available online 27 September 2008

Keywords:Arid landsDesertificationDisturbance regimesFireGrazingLoggingSemiarid landsSpatial heterogeneity

* Corresponding author. Departamento de DeAmbiental, Instituto Argentino de Nivologıa, Glaciol(IANIGLAdCONICET), 5500 Mendoza, Argentina.

E-mail address: villagra@mendoza-conicet.gov.ar (

0140-1963/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.jaridenv.2008.08.002

a b s t r a c t

The complex interactions between human activity and natural processes determine non-linear dynamicsin ecosystems that can difficult their management. Human settlements in arid lands contribute to themodification of disturbance regimes, including the introduction of new disturbances and the eliminationof others. In consequence, they can alter the functional mechanisms that allow systems to overcomelimiting factors, leading to desertification. In this revision, we evaluated the effects of the changes ondisturbance regimes produced by the different forms of land transformation on the structure andfunction of ecosystems of the Monte Biogeographical Province, in Argentinean arid west. Twoapproaches were used: the analysis of land use history and the analysis of the effects of the maindisturbances on the dynamics of different communities. We concluded that throughout the history of theMonte Desert, the joint action of natural and anthropic agents has resulted in complex dynamics thatlead most area of the Monte to a moderate to severe status of desertification. The modification of thedisturbance regime had strong consequences for several aspects of the dynamics of communities, such asspecies composition and diversity, water dynamics, soil conditions, trophic structure and productivity ofMonte Desert ecosystems. However, disturbance regimes could be managed to promote favorabletransitions in ecosystems and, therefore, could be a tool for optimizing productivity of agro-ecosystems,and recovering and conserving natural ecosystems.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The complex interactions between human activity and naturalprocesses determine non-linear dynamics in ecosystems, withthresholds, feedbacks, time lags and legacy effects. When thiscomplexity is not understood, surprising results can difficult themanagement or conservation of ecosystems (Liu et al., 2007). Inarid and semiarid ecosystems, changes of disturbance regimes,including the introduction of new disturbances and the eliminationof others, can alter the functional mechanisms (e.g. positive feed-backs between species) that allow systems to overcome limitingfactors. In such cases, land degradation processes are difficult torevert (Sole, 2007). This loss of resilience, called desertification,

ndrocronologıa e Historiaogıa y Ciencias Ambientales

P.E. Villagra).

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reduces the potential land productivity, and consequently the lifequality of local people (Vogel and Smith, 2002). Traditionally, there-establishment of historical abiotic conditions and the protectionof ecosystems against disturbances has been suggested as a neces-sary management tool to halt land degradation and to promote thenatural recovery of degraded areas (succesional-based manage-ment) (Suding et al., 2004). However, several studies indicate thatthe effects of excluding disturbances are not easy to predict.Therefore, greater knowledge of interacting factors conducingsuccession and threshold occurrence are needed to generateeffective models applicable for management and restoration ofnatural systems (Bestelmeyer, 2006; Briske et al., 2005).

Different frameworks have been proposed to assess themanagement of these complex systems based on state and transi-tions, thresholds and rangeland health models (Briske et al., 2005;Herrick et al., 2006; Westoby et al., 1989). These models understandthe dynamics of ecosystems as a set of possible stable states andtransitions among states, and interpret the ecosystem changes asa consequence of the interactions among several factors, including

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211 203

disturbance regime, weather variability, soil conditions, andmanagement. From a decision-making point of view, the knowl-edge of the possible states and transitions allows the recognition ofopportunities to achieve favorable, or avoid hazardous, transitions(Bestelmeyer et al., 2003; Briske et al., 2005).

The Monte Biogeographical Province (so-called ‘Monte’ or‘Monte Desert’, Fig. 1) occupies approximately 460 000 km2 of theArgentinean arid west (Cabrera, 1976; Rundel et al., 2007).Climatically, it is an arid to semiarid region, with mean annualprecipitation ranging from 30 to 350 mm, and temperaturesranging from mean maximum of 25.2 �C and mean minimum of10.2 �C in its northern part, to 20.4 and 7.3 �C in the southernportion. Five natural areas have been identified for the Monteaccording to the endemic assemblages (Roig-Junent et al., 2001).Most of the ecological studies have been performed in the Northern(25–30�S), Central (30–37�S) and Southern Monte (37–43�S), thethree most extensive natural areas.

Fig. 1. Geographic location and present land use in the Monte Desert biome (Argentina). TVegetation Continuous Fields product by Hansen et al., 2002). Isohyet distribution for the pnames of the political provinces mentioned on the text are also included.

Several areas of the Monte present a moderate to severe degreeof desertification, and human activities and their associateddisturbances have been suggested as the main causes of degrada-tion processes (Roig et al., 1991; del Valle et al., 1998). The maindisturbances in this region are fires, occurring from immemorialtimes to present, and domestic grazing and tree cutting, whichstarted about two centuries ago as a consequence of colonizationand economic development (Abraham and Prieto, 1981; Defosseet al., 2003; Rostagno et al., 2006).

We proposed that the current spatial patterns of Montecommunities, their present land use and conservation status is theresult of the complex interactions between natural conditions anddifferent land use histories. In order to understand these complexinteractions, we evaluated the effects of the changes on disturbanceregimes produced by the different forms of land transformation onthe present structure and function of the Monte ecosystems. Weperformed this analysis using two main approaches: (a) The

he four land use classes were elaborated from the Global Land Cover Facility (MODISeriod 1950–2000 (taken from the world climate database, Hijmans et al., 2004). The

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211204

analysis of land use history and its relation with the temporal andspatial pattern of main disturbances of the Monte; and (b) Theanalysis of the effects of the main disturbances on the dynamics ofthe different communities, focusing on the structural and func-tional changes conducting successional processes and transitionsamong different community states. We expected that this analysiswill allow us to identify typical pathways of dryland change, todetermine indicators of rangeland conditions, risk situations andmanagement opportunities, and to detect knowledge gaps fordeveloping dynamic models of the Monte ecosystems.

Fig. 2. Land use history in the Monte showing the different changes on disturbanceregime along the time. The distribution of the different processes in the vertical axisindicates the latitude where each process was more obvious within the Monte Desert.

2. Relationship between land use and disturbance regime

2.1. Brief history of main changes in land use

Historical studies pointed out the different human settlementperiods in the area and allowed us to determine the principaldisturbances occurring in the Monte and to understand the mainchanges in their regimes (Fig. 2). In the Central Monte, there areevidences of land occupation by groups of hunters–gatherers fromat least the last 10,000 years (Lagiglia, 1994). When the Spaniardsarrived (ca. 1530), the region was settled following a riparianoccupation pattern, with a relatively low impact on the environ-ment. At the end of the XVIIIth century, a great anthropic pressureon the environment begun and, since the early XXth century, theeconomy was based on intensive agriculture on highly productiveirrigated valleys. The majority of the non-irrigated areas were usedfor extensive livestock raising (Abraham and Prieto, 1999; Prietoand Abraham, 1998).

From an economic point of view, the most important plantcommunity in Northern and Central Monte region has been theopen woodland of Prosopis flexuosa (‘algarrobal’). According to Roig(1993a), the higher degradation of these woodlands occurredduring the first decades of the XXth century associated to therailroad expansion. During that period, the extraction of P. flexuosafor firewood, charcoal, and the production of gas for urban lightingincreased. During the major expansion of vineyards (1940–1960),woodlands were cut to make the posts to conduct vines. Also, asthey occupy areas with an extra water supply, such as undergroundwater, vineyards also expanded into these woodlands. A period ofexpanding market demand of timber for furniture and floor-woodfollowed, increasing the pressure for selective cut of P. flexuosa,mainly in the northern part of its distribution (Fig. 2).

In the Southern Monte, the history of use of the Monte vege-tation had a strong influence in changing disturbance regimes.Related to fire, three periods could be identified, in which thisdisturbance played a different role in relation to rangelanddynamics (Fig. 2) (Defosse et al., 2003). The first period could betraced back from ancient times and up to the end of the XIXthcentury. During this period the indigenous people frequentlyburned the shrublands for hunting, communication or otherpurposes (Claraz, 1988). The second period could be dated from theintroduction of domestic livestock (mainly sheep) at the end of theXIXth century and up to the decade of the 1980s. During this period,dramatic changes occurred in the use of these shrublands, with theintroduction of a new disturbance (grazing) (Ares et al., 1990;Soriano, 1983) and the disruption in the fire frequency (Defosseet al., 2003). The beginning of the third period could be establishedat the end of 1980s of the XXth century when continuous grazingpressure begun to diminish as a consequence of the fall of woolprice below the limits of profitability, coupled to rangelanddegradation caused by overgrazing (Ares et al., 1990). The recoveryof grasses during this period allowed the accumulation of deadgrass biomass and, as a consequence, the frequency and magnitudeof fire events increased (del Valle et al., 2004).

In summary, the history of use differed among the differentnatural areas of the Monte and was strongly related to social,economical, cultural, historical and climatic factors. As a conse-quence, changes on disturbance regime and important processes ofland transformation occurred along the whole Monte, which inturn determined changes in land use (Fig. 2). The most commondisturbances have been deforestation, grazing and fires in the non-irrigated areas and the replacement of natural ecosystems bycroplands in irrigated oasis. The relative importance of eachdisturbance has changed along the time in an independent way ineach natural area, and at least some modifications of the ecosystemstructure appear to have been governed by these changes.

2.2. Present land use

We identified and categorized several natural and man-devel-oped features of the Monte Desert biome in terms of land coverusing the hierarchical classification system developed by Andersonet al. (1976). We only analyzed the two first levels (lower resolu-tion) given the extension of the region studied. We used theinformation provided in the Global Land Cover Facility (MODISVegetation Continuous Fields product by Hansen et al., 2002) andthe units of soil classification and mapping taken from ArgentinaSoil Map (Godagnone et al., 2001).

The analysis classified present land use of the region in fourclasses (Table 1 and Fig. 1). The Agricultural land class is mainlyfound in alluvial plains, with soils having very low limitations.Although this class supports the highest human pressure,management and conservation practices allow to maintain itsproductivity. Irrigation agriculture is the main activity, althoughsome livestock activities are also carried out, alternating cropproduction with the cultivation of some pluriannual pastures uponwhich livestock raising is based. The majority of the Prosopiswoodlands located in low lands are used as timber areas, althoughit is now a secondary activity as compared with livestock raising.

Rangeland classes 1 and 2 are mainly used as livestock raisingareas. Rangeland class 1 is attributed to lands on which livestockproduction is based on cultivated pastures combined with shortrotation of annual crops. It comprises lands with low to moderatelimitations in soil water or nutrients availability, which require theapplication of special soil and water conservation and managementpractices to maintain their productive levels. Rangeland class 2corresponds to the majority of the Monte region (Table 1 and Fig. 1)

Table 1Present land use in the Monte Desert biome (Argentina), showing different units, levels and surface area

Level I (see Fig. 1) Level II Surface

Ha %

Agricultural land � Cropland and pasture� Orchards, groves, vineyards, horticultural areas� Deciduous forest land� Open Prosopis woodlands (Algarrobales)� Mixed forest land

5,068,100 10.8

Rangeland 1 � Grass steppes� Shrub grass steppes� Pasture

4,447,775 9.5

Rangeland 2 � Shrub steppes/Larrea shrublands� Shrub steppes with grasses

23,604,450 50.5

Barren land � Bare exposed rock/rocky slope vegetation� Sandy areas/sand dunes communities� Dry salt flats/halophytic communities� Littoral environments� Strip mines, quarries and gravel pits� Transitional areas� Mixed barren land

13,653,050 29.2

46,773,375 100

Level I shows the four classes of land use classified using MODIS Vegetation Continuous Fields product (Hansen et al., 2002). Level II shows the different vegetation formationsincluded in each class.

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and includes areas where soil and climate conditions only allow itsuse as natural pastures for livestock raising. Related to this, thecarrying capacities the different rangelands of the Monte cansustain have been analyzed by Guevara et al. (2008). The Barrenland class is presented in fragmentary form, generally associated toone of the classes previously described.

2.3. Desertification status

Two studies have described the desertification status with anintegral approach for the Central Monte (Mendoza Province) (Roiget al., 1991) and Southern Monte (del Valle et al., 1998). Both studiesincluded in their analysis the fragility grade (inherent risk) of thesystem to desertification (amount of all indicators that come fromphysical and biological support), and the degree of human pressurethese ecosystems may withstand (amount of all indicators relatedto the demand of resources made by social groups). In both sites,almost all the area under study presented a moderate to severedegree of desertification, attributable to an interaction betweensystem’s own fragility and anthropic actions. Consequently, naturaldesert-type complexes started to develop in it.

According these studies, aridity, water and wind erosion, andsalinization processes are the main factors contributing to systemfragility; while livestock pressure, logging and fire regime modifi-cation are the main anthropic pressures in the non-irrigated areas.The ecosystems bearing more human pressure are areas with lowpopulation density but offering resources such as P. flexuosa andgood pasturelands. In the Central Monte, the problems concerningland tenure, the isolation and social rejection of the inhabitants ofthe desert, caused strong exodus and rural migration. The aban-donment of productive lands coupled to the loss of traditionalvalues, led to an increase of sub-urbanization processes in theurban outlying areas, with settlements that put pressure on fragileareas with a growth of socially rejected sectors, lack of security andsocial exclusion (Roig et al., 1991). Although these studies only

comprise a relatively low portion of the Monte region, similarsituations are expected to occur in the rest of the Monte region.

3. Disturbances as ecological factors

3.1. Fire effects on ecosystem dynamics

During 2004 and 2005, 1338 and 1521 fire events were detected,respectively in the Monte (Fig. 3). The highest density of fire eventsoccurred in areas with annual precipitation between 200 and300 mm and the lowest density in area with annual rainfallbetween 100 and 200 mm (Table 2). Among vegetal communitiesdefined by Eva et al. (2004) closed shrubland steppes (Larreashrublands) appeared as the physiognomic group most affected byfire events (nearly 60% of the fire events occurred in this group).

Several studies about the role of fire on grasslands and shrub-lands all over the world showed the strong effects of fire on func-tional processes of these ecosystems. In the Monte, the immediateeffect of fire is the drastic reduction of biomass, followed by therelatively quickly recovery of the herbaceous strata and the slowrecovery of the shrubby strata thereafter (Guevara et al., 1999;Passera et al., 2007; Rostagno et al., 2006). The rate of post-firerecovery of different communities of the Monte is relatively slow.Differences between burned and non-burned communities couldbe observed 15, 12 and 11 years after wildfire events in GuadalPlateau (Mendoza) (Passera et al., 2007), in Nacunan Reserve(Mendoza) (Rossi, 2004), and in the Southern Monte (Rostagnoet al., 2006), respectively. In Nacunan, a drastic reduction in grass,shrub and tree cover was detected 6 month after a fire, whileannual forbs showed an increase in this attribute (Marone, 1990).Twelve years later, a new recovery state was observed, with shruband grasses reaching 50% of the cover, coincidentally with a steadydiminution of annual forbs (Rossi, 2004).

This typical post-fire successional pathway in these shrublandshas led to think on prescribed fire as a management tool to revertchanges produced by overgrazing. In general, overgrazing produces

Fig. 3. Maps of the Monte Desert showing hot spots detected by MODIS Terra/Aqua satellites in 2004 (a) and 2005 (b).

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changes in the communities, increasing the amount and cover ofshrub biomass and reducing that of grasses, trend that many timescannot be reverted solely by the exclusion of grazing (Beeskowet al., 1995; Bertiller and Bisigato, 1998). Rostagno et al. (2006)interpret the interaction between both disturbances in theSouthern Monte in the context of a steady-state model where theshrub-dominated steppes appears as a stable community. In thiscondition, fire would trigger the transition to a grass-dominatedstate, in which its persistence will depend on the recovery rate ofdominant shrub species. Despite grass steppe is not a true stablestate, the recurrence of fire could maintain the system in a slowlychanging grass-dominated unstable state. Intense grazing pressurewould reduce fine fuels and, consequently, modify the fire regime.This type of dynamics has been described for other semiarid lands

of the world (Archer, 1995). However, in more xeric areas, thepresence of woody species appear to be necessary for the estab-lishment of grass species (Aguiar and Sala, 1999) and, therefore, theeffect of fire-eliminating woody species could imply a decrease ingrass productivity.

From the perspective of sheep production, the model proposedby Rostagno et al. (2006) pointed out that fire improve grasslandcondition. However, different history of use, fires with differentseverity and time of occurrence, all combined with the variability ofrains, and the structure and composition of the communityinvolved, makes the post-fire vegetation responses difficult topredict. Bran et al. (2007) found that the post-fire dynamics in theSouthern Monte depends on the severity of the event, describinga faster grass and slower shrub establishment at lower fire

Table 2Proportion of hotspots obtained from MODIS in areas with different mean annualrainfalls in 2004 and 2005

Mean annual rainfalls Surface (km2) Surface percentages Hotspotspercentages

2004 2005

<100 1505 0.3 0.0 0.0100–200 212,916 45.6 34.7a 27.0a

200–300 158,820 34.0 54.9b 59.7b

300–400 77,646 16.6 9.9a 20.3>400 16,270 3.4 0.5 6.7

A proportion comparison test was performed for each rainfall class.a Indicates percentages of hotspots lower than expected according to surface

percentages.b Indicates hotspots percentage higher than expected.

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211 207

intensity. Kropfl et al. (2007) described a post-fire reduction of thecryptogamic crust. Guevara et al. (1999) showed differences inpost-fire succession if the community is subjected to grazing, withpositive effects such as a diminution of unpalatable grasses, andnegatives as an increase in the proportion of bare soil patches.Passera et al. (2007) found a diminution in carrying capacity aftera fire in an Atriplex lampa community, due to a reduction in forageprovided by shrubs. These examples pointed out that in order touse prescribed fire as a management tool to improve rangelandcondition, it has to be preceded by detailed studies about theecology of the fire in that system and its consequences in ecosystemfunction (Defosse and Urretavizcaya, 2003).

Among the indicators to analyze for a better understanding ofpost-fire succession, biological interactions, diversity, and changeson the physical conditions of environment have been analyzed indifferent areas of the Monte. In Nacunan Reserve (Central Monte),differences in the structure of vegetation patches were observed ina burned allotment, with higher species concentration below Larreashrubs than in the nearby unburned areas. This fact could indicatea facilitator role of Larrea shrubs in the establishment of grasses andforbs in the first successional stages after a fire event, although in itsmature stages, this community showed strong signs of competitionamong Larrea shrubs and grasses (Cavagnaro and Passera, 1991;Rossi, 2004).

Changes in vegetation produced by fire affect wildlife of theMonte region in different ways. While species richness of smallmammals could diminish (from five species to one) in burned areas(Ojeda, 1989), a concomitant increase could occur in the relativeabundance of species having ecomorphological characteristicssuitable to exploit the open patches left by the fire (i.e. Eligmodontiatypus, and Lagostomus maximus) (Kufner and Chambouleyron,1993; Ojeda, 1989). Fire affects bird richness more than doesgrazing, being this effect related to the reduction of plant cover(mainly woody and perennial grasses) caused by the fire (Milesiet al., 2002). Right after a fire, the immediate response of birdsimplies losses in their local diversity, since burned sites are avoidedby the highest richness bird guilds (e.g. foliage insectivorous),although preferred by other small guilds (e.g. long-fly insectivores).At regional level, the patchiness created by burned and unburnedsites increases species diversity, this being a consequence of thecomplementary spatial distribution among species harmed orbenefited by fire (Marone, 1990).

Soil erosion is one of the main degradation processes in range-lands of the Southern Monte (Rostagno et al., 1999; Sunico et al.,1996). In these rangelands, the small-scale landscape mosaic iscomposed of two patch types: individual or aggregated shrubsgenerally associated to mounds of higher fertility, and intershrubareas with bare soil or a low herbaceous cover and lower fertility(Rostagno et al., 1991). This contrasting pattern in soil resourcesmay contribute to maintain the shrub dominance by increasing the

amount of water percolation to deeper soil horizons (Bertiller et al.,1991; Rostagno et al., 2006). By diminishing vegetation and littercover, wildfires can trigger dramatic increases in wind and watererosion and may disrupt this pattern of mounds–intermoundsareas. In mound microenvironments, the mean remotion of sedi-ments after a fire occurred in January 1994 was 130 ton year�1 ha�1,and 114 ton year�1 ha�1 after a fire occurred in 2001(Rostagnoet al., 1999, 2002). Similar amounts of sediment remotion by watererosion were registered by Martınez Carretero (1983) in the Andeanpiedmont of Mendoza.

One of the consequences of soil erosion in burned areas is thechanges in soil surface characteristics due to differential removaland sedimentation of soil fractions. Gravel (fraction >2 mm), verycoarse sand (1–2 mm) and part of the coarse sand (0.5–1 mm) arenot removed; gravels form the desert pavement that covers part ofthe eroded mounds. Very fine, fine and medium sand fractions(0.05–0.5 mm) are re-deposited within the burned area, formingsmall accumulation tongues that increase ecosystem heterogeneity.These new patches are readily colonized by Poa lanuginosa,a rhizomatous grass that grows on loose, sandy soils (Beeskowet al., 1995). On the contrary, part of the very fine sand and morethan 90% of clay and silt is removed by winds outside the burnedarea as suspended material, originating the frequent dust storms.The recovery of vegetation cover, mainly by herbaceous perennials,slows down the erosive process.

3.2. Grazing effects on spatial heterogeneity and biological diversity

Extensive livestock grazing (sheep, cow and goats) occurs onabout 60% of the Monte land area and has important consequencesin shaping and modifying landscape structure and species diversity(Guevara et al., 2008). The most obvious and extensive effects ofgrazing on vegetation structure are the reduction of total plantcover and grass strata, and the increase of bare soil patches. Incontrast, the direct effects of grazing on shrub abundance differamong sites and grazing intensities (Bisigato and Bertiller, 1997;Kropfl et al., 2007; Rossi, 2004). However, and as mentioned in theSouthern Monte, indirect effects of grazing (reducing the amount offine fuels that carries the fire) favor shrub expansion througha reduction in fire frequency (Rostagno et al., 2006).

The changes in vegetation pattern occurred after 30 years ofgrazing exclusion have been determined in the Central Monte(Nacunan Reserve, Mendoza) (Rossi, 2004). Although total speciesrichness on the whole Reserve appears to have only slightlychanged, a closer analysis of its different communities showed thatspecies diversity and richness have increased in most of them, ascompared with a previous state in which grazing was permitted(Rossi, 2004). However, and together with this recovery process,there exist some evidences that grazing exclusion induces a spatialhomogenization at the landscape scale. This means that the strongheterogeneity that existed previous to grazing exclusion (Roig,1971) tended to diminish, mainly due to colonization, distributionand expansion of some species through time. The most homoge-neous distribution of accompanying species could be the result ofa lower environmental stress (exclusion of grazing and higherprecipitations), allowing their distribution to increase beyond itsoptimal habitat.

The changes produced by grazing in the vertical and horizontalpattern of vegetation (Rossi, 2004; Tabeni and Ojeda, 2005; Tabeniet al., 2007) could affect species and functional groups of nativewildlife in different ways, mainly through its effects on the avail-ability of food and safe sites for feeding and nesting (Gonnet, 2001;Milesi et al., 2002). Areas recovering from grazing favor smallrodents such as Graomys griseoflavus, Calomys musculinus y Akodonmolinae (Maldonado Curti, 1990; Tabeni and Ojeda, 2005), whichrequire complex habitats in densely vegetated patches to nest and

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211208

find refuges to avoid predators (Corbalan and Ojeda, 2004). Thelower availability of open habitats in grazing excluded sites,however, appears to affect the occurrence of species havingparticular attributes to escape and with well-developed commu-nication skills (i.e. long legs, hair-padded feet, jumping skills, light-colored, well-developed sensorial organs). That is the case ofDolichotis patagonum and Eligmodontia typus, which markedlyprefer open areas produced by grazing (Kufner and Chambou-leyron, 1991; Tabeni, 2006).

In Southern Monte, sheep concentration around watering pointsproduces a degradation gradient in which it is possible to deter-mine changes at both vegetation and wildlife levels (Bisigato andBertiller, 1997). Sheep activity diminishes as the distance to thewater point increases, producing a vegetation pattern in whichgrasses and palatable shrubs are reduced in the water hole neigh-borhood. As a consequence, some native mammals such as theguanaco (Lama guanicoe (Muller 1776)) adopt a pattern of use,which is the reverse of that of sheep. This fact would in turndiminish the potential trophic competition between these twospecies (Saba et al., 1995).

Microenvironmental variation caused by grazing has a signifi-cant effect on the invertebrate fauna. From a functional point ofview, the most abundant groups in grazed areas are composed ofinsectivores, herbivores and predators, while detritivorous (espe-cially Tenebrionidae) considerably decrease. It has been suggestedthat the impoverishment of this group in disturbed areas couldindicate a functional alteration in that ecosystem. This responsecould be linked to lower food availability because of a loweraccumulation of litter, or due to a lack of refuge areas to avoidpredators. These environmental conditions would also explain thehigher presence of predators such as different spiders, which wouldbe benefited by a better visualization of the preys (Lagos, 2003).

The long-term effects of the functional changes produced bygrazing in Monte ecosystems are largely unknown. However, theevidences of increasing spatial heterogeneity at moderate grazingpressure in relation to livestock exclusion, suggest that grazinggenerate a mosaic of habitats, which determine the structure andcomposition of species assemblages in response to different lifehistories of species.

3.3. Wood clearcuting in the Monte region

Three main processes have occurred in relation to the use ofProsopis woodlands in the Monte: (a) clearcut for the use of fire-wood and vineyard posts; (b) selective logging for timber produc-tion; and (c) woodland replacement for urbanization andcroplands. These interventions not only decreased Prosopis wood-lands extension, but also simplified the structure of its communi-ties. In the northern area, the selective cut of healthier and wellformed individuals led to a genetic erosion (Roig, 1993b). Theurbanization and irrigation processes led to a reduction in size ofwoodlands, increasing at the same time the demand of forestproducts from the remaining woodlands. The replacement processusually implied the complete elimination of all vegetation. Today,more than 20% of the woodlands have been replaced by croplandsin the Northern Monte valleys. Since P. flexuosa needs an under-ground water complement to grow in the Monte, no expansionover new areas have been observed for this species.

The cut of P. flexuosa has immediate effects on communitystructural attributes, such as the drastic reduction on wood biomassof single-stem trees, followed by a rapid resprout from theirstumps, changing consequently tree habits. The large number ofstems per tree (until 11) and the large crown diameters observed atNacunan Reserve (Mendoza) reflect the vigorous stump resprout-ing after these woodlands were logged on the first decades of theXXth century (Villagra et al., 2005b).

Experimental studies aimed at analyzing other effects oflogging on the functioning of Monte ecosystems are scarce. It hasbeen demonstrated, however, the importance of P. flexuosa as themain structural species in these ecosystems. P. flexuosa has anessential role in maintaining diversity at regional scale, generatingspatial heterogeneity in edaphic and microclimatic conditions(Rossi, 2004; Rossi and Villagra, 2003), offering nesting andfeeding areas for granivores and insectivorous birds (Cueto et al.,2006) and also for epigeous insects (Lagos, 2003). These factsmake us to postulate that the elimination of P. flexuosa must affectsome functional processes of these communities, namelyproductivity and diversity. In some Prosopis woodlands of the aridChaco, related evolutively with those of the Monte region, it hasbeen observed that tree logging reduces species richness anddiversity, and although the development of shrubs are favored(Moglia and Jofre, 1998), the end result is a general loss in thecarbon stocks (Bonino, 2006).

The effects of the past use of Prosopis woodlands in the land-scape of the Monte are difficult to revert as a consequence of thelow rate of regeneration and growth of this species (Villagra et al.,2004). This low rate of regeneration would be related to thetemporal variability of seed production, the seed and seedlingpredation, and the low frequency of occurrence of the climaticconditions promoting its establishment (Villagra et al., 2002, 2004).In addition, the productivity of Prosopis woodlands in three sites ofthe Monte Desert (Pipanaco, 27�580S; 66�110W; Telteca, 32�200S;67�520W; and Nacunan, 34�030S; 67�580W) suggested that thelength of the biological cutting cycle necessary to replace biomassloss after cuts is longer than the one presently used (Villagra et al.,2005a).

Shrub elimination through different techniques generatesdifferential growth responses for different life forms. In the CentralMonte, mechanical shrub control may increase grass cover,resulting in an increase of the carrying capacity of some commu-nities (Passera et al., 1992). This effect is more evident in Larreacuneifolia communities, with poor grass coverage, than in Larreadivaricata communities, in which grasses are more abundant(Passera et al., 1996). Similar results were obtained in the SouthernMonte (Kropfl et al., 2007), suggesting that this would be a commonbehavior at least in sites with mean annual rainfall over 300 mm.Based on 14 years of experience in Nacunan, Allegretti et al. (1997)concluded that the only strong, long-lasting effects on woodyvegetation resulted from root-plowed areas. For herbaceousspecies, the general trend was an increase, especially in hand-cutting and root-plowing treatments. The increase in grass cover inthis case may be attributed to the elimination of competition forwater, as demonstrated by Cavagnaro and Passera (1991). Althoughno studies have been conducted relating the effects of shrub controlon the growth of tree species in the Monte, an increase in the radialgrowth of P. flexuosa has been observed following shrub control inthe dry Chaco (Carranza et al., 2000).

3.4. Other disturbances

Other kind of disturbances occurs in the Monte, such as flood-ing, hailstorms or road opening, and also due to soil movementgenerated by mining operations. Although these disturbances areless extended than the ones earlier described, they also pose somecontrol on community dynamics and generate heterogeneities atlocal scale (Bisigato et al., 2008). The long-term effects of thesedisturbances on the ecosystem dynamics are almost unknown. InCentral Monte, it has been observed that sporadic flooding due toriver overflows may control the dynamics of riparian areas, causingmortality of species intolerant to inundation, allowing at the sametime the establishment of pioneer species, and modifying theedaphic conditions through deposition of sediments and salts

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211 209

(Villagra and Roig, 1999). The temporal persistence of thesecommunities would depend on the recurrence of the floodings.

4. Implications for the sustainable management of naturalareas

The severe ecological situation showed by the desertificationmaps (Roig et al., 1991; del Valle et al., 1998) calls for thedevelopment of a general strategy to overcome the desertificationprocess. This strategy should consider the biological potentialof ecosystems (e.g. forage or wood productivity), the response ofecosystem processes to disturbance regimes, and the importanceof landscape diversity and environmental heterogeneity in themanaging of natural resources.

Adoption of state and transition models would be a usefulframework to detect what information is necessary to identifyundesired ecosystem states and early signs of degradation, and topromote favorable transitions (Bestelmeyer et al., 2003; Briskeet al., 2005). Only a few studies have studied the ecosystemdynamic using this approach in the Monte (Rostagno et al., 2006).However, numerous studies focusing on the relation betweendisturbances, land use and ecosystem response allow the identifi-cation of main pathways among different states and constitutea good basis to the construction of dynamics models. Moreover, aswe discussed above, several research lines have advanced on theknowledge of the effects of disturbances on taxonomic groupsdifferent than plants, interactions among species, and trophicdynamics, and on hydrologic and geomorphologic processes. Theseadvances give the opportunity to include in the models aspects ofthe functional dynamics that have been rarely assessed in state andtransitions models (Bestelmeyer, 2006).

Throughout the history of the Monte Desert, the joint action ofnatural and anthropic agents has resulted in complex dynamicsthat make the ecosystem responses to management practicesdifficult to predict through the traditional view of rangelandmanagement. Disturbances are important factors modeling land-scape along the whole Monte Desert. The strong modifications ofthe disturbance regime produced by social, economical, culturaland climatic factors have determined dramatic changes of theecosystem dynamics, which in turn have socio-economicalconsequences and determined changes in land use (Fig. 2). Whatis more, understanding the functional consequence of distur-bances or its exclusion has importance from a point of view ofconservation. A strong relation among disturbance, heterogeneityand biodiversity has been described in the Monte Desert. It isclear that the exclusion of disturbances does not conductnecessarily to higher diversity at regional scale. In contrast, theenvironmental heterogeneity appears to play a major role inspecies responses to landscape changes and, consequently, theconservation of regional diversity would be related with themultiscale, from local to regional, variability in the disturbanceregime. This is consistent with different studies in other regionswhere communities under restoration showed unpredictableresponses to management efforts and the unprotected ordisturbed areas seemed to play an important, but largely ignored,role in the conservation of biological diversity (Fabricius et al.,2003; Suding et al., 2004; Verdu et al., 2000).

In this context, the present land use and the conservation statusof the Monte is the product of the different disturbance histories ofeach site. Grazing is today the most extended human activity andthis disturbance has been interacted with others in different ways.In the Northern Monte, logging activities reduced the extensionand simplified the structure of P. flexuosa woodlands, and thesedegraded areas were therefore transformed into pastoral areas(Fig. 2). The effects of the interactions between both disturbanceson the recovery and conservation of the woodlands are unknown

and need new research in order to propose strategies for achievingan optimal balance between production and conservation.

The interaction between fire and grazing was more studied,especially in the Southern Monte. In general, fire and shrub controlfavor grass-dominated communities while grazing indirectly favorsshrub encroachment. The balance among these disturbancesappears to be the trigger of transitions between ecosystem states.However, the high environmental heterogeneity of the Monteobserved at different scales (Bisigato et al., 2008), the differentresponses described in different areas for the same disturbance,and the different land use histories among them, suggests thenecessity to place the models not only in the context of edaphicsand vegetation conditions, but also in the different structural andfunctional thresholds present in each area.

But not only is the anthropic pressure important in the modi-fication of disturbance regime. As climate and local weather are animportant factor controlling fire regime (Defosse et al., 2003;Dentoni et al., 2001), it could be expected that global climaticchange would affect or modify in some way fire frequency andintensity in the Monte. Among the changes for the region, increasesin the average precipitation (especially in the north and centralportion of the Monte), in mean temperatures, and in interannualvariability of both variables are expected (Labraga and Villalba,2008). These scenarios suggest that global climatic change wouldfavor an increase in fire frequency for the Monte region asa response to increasing grass productivity, climatic variability andthe probability of electric storms. This implies that some woodycommunities could be transformed to ecosystems with simplifiedstructure in several areas of the Monte. It will be important,however, to understand the dynamics of the communities underthe new scenario in combination to other disturbance such asgrazing, since the simplified structure could be a grass-dominatedcommunity with higher carrying capacity or a degraded systemwhere all strata are reduced.

The stability of the different community states is not very wellknown in the Monte. In general, degraded communities have beenconsidered as variation in the different steps of their own ecologicalsuccession (Dalmasso, 2006). However, our analysis suggested that,when the impact of disturbances is high, the necessary time torecover may be too long for human-time scales. In these cases, it issuggested the application of management practices (i.e. restorationactivities) to accelerate the transition to desired community states.However, the development of restoration techniques for the Monteregion is still incipient, but first experiences implies the knowledgeof functional threshold to trigger transitions. Dalmasso (2006)made the first revegetation trials in oil producing areas, proposinga phytosociological analysis with a successional point of view withthe objective of selecting plants that, according to dynamichypotheses, allowed to accelerate natural vegetation recovery. Hisstudy presented evidences showing that planted species facilitatethe establishment of L. divaricata and L. cuneifolia, which arecharacteristics of the mature community. Guevara et al. (2005)proposed the seeding of weeping lovegrass, Eragrostis curvula, asa useful tool for rehabilitation of degraded rangelands in the Monteand Espinal biomes. In this case, the objective of the rehabilitationwas to improve grazing carrying capacity, being economicallyfeasible in most of the analyzed scenarios. Fire, as a managementtool, could be a good option in some cases, and a risky activity inothers, especially is if its role in that system is poorly understood,and also if rules and regulations of use are not complied withRostagno et al. (2006).

The need for reverting the socio-economic situation of Montedwellers and the process of environmental degradation has lead tothe development of programs for the integral management ofmultipurpose trees, especially for the two dominant Prosopisspecies in the Monte (P. chilensis and P. flexuosa). In order to develop

P.E. Villagra et al. / Journal of Arid Environments 73 (2009) 202–211210

adequate technologies for reforestation programs in the Monte, thegenetic variability of P. flexuosa and P. chilensis has been evaluatedin common garden assays (Cony, 1996). However, the technologiesfor reforestation of degraded areas in the Monte have not been yetadequately developed.

Early studies that allowed a proximity to the current state ofdegradation and desertification of the Monte considered physical,biological, socio-economic and political indicators. However,a temporal analysis is essential to identify causes and evolution ofthe processes. The interdisciplinary contributions of environmentalhistory and auxiliary disciplines such as dendrochronology, pal-inology, archeology and anthropology could help in the establish-ment of a temporal axe through identifying a time-base line(Abraham and Prieto, 1991, 1999; Prieto et al., 2003).

We can conclude that the great challenge posed to scientists andextensionists is to elaborate management proposals for naturalecosystems, sustainable in time and focused on the promotion offavorable transitions. In that sense, the management of distur-bances regimes is not only a tool for optimizing productivity ofagro-ecosystems but also for recovering and conserving naturalecosystems. The modification of disturbance regimes, however,could trigger desertification processes, with the consequent dimi-nution of productivity and negatively affecting the quality of life oflocal people.

Acknowledgments

This review was supported by the National Agency for Scientificand Technological Promotion of Argentina (PICT 01-11050 and PICT13-15034), CONICET (PIP 5953; PIP 6413 and PIA 6387/97), andGOFC-GOLD, Maryland University, RedLaTif (AQL2004).

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